Simulating Heme Using DFT+U: A Step Toward Accurate Spin-State Energetics.

D. A. SCHERLIS, M. COCOCCIONI, P.SIT, N. MARZARI

JOURNAL OF PHYSICAL CHEMISTRY B - (Print)

AMER CHEMICAL SOC

Año: 2007 vol. 111 p. 7384 - 7391

1520-6106

We investigate the DFT + U approach as a viable solution to describe the low-lying states of ligated andunligated iron heme complexes. Besides their central role in organometallic chemistry, these compoundsrepresent a paradigmatic case where LDA, GGA, and common hybrid functionals fail to reproduce theexperimental magnetic splittings. In particular, the imidazole pentacoordinated heme is incorrectly describedas a triplet by all usual DFT flavors. In this study, we show that a U parameter close to 4 eV leads to spintransitions and molecular geometries in quantitative agreement with experiments and that DFT + U representsan appealing tool in the description of iron porphyrin complexes, at a much reduced cost compared to correlatedquantum-chemistry methods. The possibility of obtaining the U parameter from first principles is exploredthrough a self-consistent linear-response formulation. We find that this approach, which proved to be successfulin other iron systems, produces in this case some overestimation with respect to the optimal values of U.